This invention relates to correction methods for correcting distortions, especially spool distortions, of a screen image of the X-ray radiograph diagnosis devices by which a medical diagnosis can be effected with high accuracy on the basis of the radiographs of high quality.
Generally, in the case of the medical diagnosis based on an X-ray radiograph imaged displayed on screen, an object is positioned between an X-ray tube and an X-ray image intensifier. An X-ray transmitted through the object is detected and converted into a digital signal. The diagnosis is effected on the basis of the digital signal. However, the image obtained from the X-ray image intensifier generally contains a geometric distortion peculiar to an electron lens system. The distortion is referred to as the spool distortion since a square is distorted into the form of a spool. Since the distortion impairs the geometric accuracy of the X-ray image, various correction methods have hitherto been proposed.
For example, FIG. 5 is a diagram showing a structure of a conventional X-ray radiograph diagnosis device, which is disclosed in Japanese Laid-Open Patent (Kokai) No. 2-10636. In FIG. 5, the X-ray R is exposed from an X-ray tube 1 which is vertically translatable as indicated by the double-headed arrow. An X-ray image intensifier 2 disposed coaxially with the X-ray tube 1 opposes the X-ray tube 1 to receive the transmitted X-ray R. An object 3 is positioned between the X-ray tube 1 and the X-ray image intensifier 2. An image processor 4 coupled to the X-ray image intensifier 2 digitizes a signal obtained by the transmitted X-ray R. A display device 5 displays the digitized screen image obtained by the image processor 4.
FIG. 6 is an axial sectional view showing the details of the X-ray image intensifier of FIG. 5. The X-ray tube focus 10 corresponds to the radiation source of the X-ray tube 1. A vacuum tube 20 accomodates the following: a photoelectric cathode 21 which generates photoelectrons E upon receiving the X-ray R transmitted through the object 3. As shown in FIG. 6 (the object 3 may, for example, be a plate-shaped object instead of a human body when a test, for example, is performed); a plurality of grid electrodes 22 converges photoelectrons E to the cross-over point P; a front stage anode 23 and a back stage anode 24 together constitute an electron lens system for photoelectrons E passing the cross-over point P; an intermediate electrode for correction 25 interposed between the front stage anode 23 and the back stage anode 24; and a fluorescent film 26 having an output surface 26a which emits light in accordance with a strength of receiving the photoelectrons E. The trajectory distance between the X-ray from the X-ray focus 10 and the photoelectric cathode 21 is represented by FID.
FIG. 7 is a diagram showing the magnitude of the spool distortion (plotted along the ordinate) in relation to distance from a distortion center (plotted along the abscissa) for various values of X-ray trajectory distance FID. The abscissa represents the distance from the distortion center of the spool distortion, where an outer radius is plotted at 100 percent. The ordinate represents an integral of the geometric distortion corresponding to the magnitude of the spool distortion. The respective curves correspond to the cases where the X-ray trajectory distance FID from the X-ray focus 10 to the photoelectric cathode 21 varies from 1000 mm to 700 mm by the step of 100 mm.
It is seen from FIG. 7 that the spool distortion increases as the distance from the distortion center (the intersection of the axis of the electron lens and the photoelectric cathode 21) increases. Further, the spool distortion increases as the X-ray trajectory distance FID becomes shorter.
Next a correction method for the spool distortion of the conventional X-ray radiograph diagnosis device is described. The X-ray R exposed from the X-ray focus 10 of the X-ray tube 1 falls on the photoelectric cathode 21 of the X-ray image intensifier 2 after transmitting through the object 3. Thus, the photoelectrons E generated from the photoelectric cathode 21 and converged by the electron lens system passes the cross-over point P and irradiates the fluorescent film 26 to form an image of the object 3. The X-ray image generated at the output surface 26a of the fluorescent film 26 is digitized by the image processor 4 and the digitized image is displayed on the display device 5.
Under this circumstance, the X-ray tube 1 may be vertically translated as shown in FIG. 5. In accordance with the variation of the X-ray trajectory distance FID, however, the magnitude of the spool distortion integral varies as shown in FIG. 7. Accordingly, the quality of the picture (especially at the periphery of the image) is injured. The diagnosis is thus very difficult. To overcome this difficulty, the voltage applied on the intermediate electrode for correction 25 is adjusted in accordance with the X-ray trajectory distance FID, such that the spool distortion integral remains constant at respective points upon the display screen. However, the spool distortion itself is not eliminated. Further, when the distortion center is not aligned with the axis of the X-ray, an asymmetric spool distortion persists.
Thus, the above conventional X-ray radiograph diagnosis device has the following disadvantage. Since only the voltage applied on the intermediate electrode for correction 25 is controlled, the spool distortion, although kept constant, is not eliminated. Thus, the diagnosis must be performed on the basis of the X-ray image containing the spool distortion. Worse still, when the distortion center is not coaxially aligned with the X-ray center, an asymmetric spool distortion persists. Then, the image is distorted asymmetrically and the spool distortion integral cannot even be kept constant.